Method and Device for Plugging of a Subsea Well

ABSTRACT

A method and a device are for plugging a submerged well in which a wellhead valve is arranged at the seabed. The method includes: moving a perforation tool into the well and perforating a first perforation in the wall of an inner casing; perforating a second perforation in the wall of the inner casing at a considerable distance from the first perforation; filling a casting substance into at least a portion of an annulus between the first perforation and the second perforation; placing a connection housing between the well and the wellhead valve; performing the first perforation through the pipe wall of at least the inner casing at a height level at which the perforation enables access to the wall of a borehole; setting a packer in the inner casing between the two perforations; connecting a stinger to the connection housing, the stinger communicating with the first perforation; circulating fluid through an annulus located outside the inner casing, between the first perforation and the second perforation to clean the annulus; and filling a casting substance into the inner casing below the packer and at least into a portion of the annulus between the first perforation and the second perforation.

This invention relates to a method for plugging a subsea well. More particularly, it relates to a method for plugging a subsea well in which there is a wellhead valve arranged at the seabed, the method including:

-   -   moving a perforation tool into the well and perforating a first         perforation through the wall of an inner casing;     -   perforating a second perforation through the wall of the inner         casing at a considerable distance from the first perforation;     -   filling at least a portion of an annulus in an area between the         first and second perforations.

The invention also includes a device for practicing the method.

Subsea wells, that is to say wells that have been drilled from the seabed, typically petroleum wells, must be securely plugged before they are permanently abandoned. In several countries there are official requirements with respect to how the work is to be carried out and to its completion.

For example, it may be necessary to fill an annulus outside a casing with a casting substance over a length of several hundred metres. In some countries, for safety reasons, the plugging work must be carried out with a wellhead valve mounted on the well.

GB patent 2414492 discloses how a plugging operation may be performed by perforating a casing and using a two-wiper plug system. The two parts of the two-wiper plug system are closed and released from a tool in a sequence by means of signals from the surface. The method according to this document is unsuitable when a wellhead valve is in position on the well.

The invention has for its object to remedy or reduce at least one of the drawbacks of the prior art.

The object is achieved in accordance with the invention through the features which are specified in the description below and in the claims that follow.

A method for plugging a subsea well in which a wellhead valve is arranged at the seabed is provided, the method including:

-   -   moving a perforation tool into the well and perforating a first         perforation in the wall of an inner casing;     -   perforating a second perforation in the wall of the inner casing         at a considerable distance from the first perforation;     -   filling a casting substance into at least a portion of an         annulus in an area between the first and second perforations.

The method is characterized by further including:

-   -   placing a connection housing between the well and the wellhead         valve;     -   performing the first perforation through at least the wall of         the inner casing at a height level at which the perforation         enables access to the wall of a borehole;     -   setting a packer in the inner casing between the two         perforations;     -   connecting a stinger to the connection housing, the stinger         communicating with the first perforation;     -   circulating fluid through an annulus located outside the inner         casing, between the first and second perforations to clean the         annulus; and     -   filling a casting substance into the inner casing below the         packer and at least into a portion of the annulus between the         first and second perforations.

If the packer forms part of the stinger, the packer is set after the stinger has been positioned in the inner casing, but before the circulation of cleaning fluid is started.

The second perforation may be close to the seabed. It is usual that the filling is performed by means of pumping.

The circulation of cleaning fluid may be carried out via the connection housing, the stinger, a portion of the inner casing, the first perforation, the annulus, the second perforation, a portion of the inner casing and back via the connection housing. The packer prevents the cleaning fluid and casting substance from flowing back via the inner casing.

The casting substance is supplied via the flow path mentioned, but is stopped when a suitable amount of casting substance has been supplied. The amount of casting substance must be sufficient to ensure that the casting substance fills the necessary length of the annulus.

The method may further include removing the packer and the stinger. These may relatively simply be pulled up after the casting substance has set and the packer has been relieved.

The method may be practised by means of a plugging tool for plugging a subsea well in which a wellhead valve is arranged at the seabed, the tool being characterized by including a connection housing which fits sealingly between the well and the wellhead valve, the connection housing being formed with a through-going axial bore, a first pipe bore extending from the outside of the connection housing into the bore, and a second pipe bore extending from the outside of the connection housing and communicating with the pipe bore of a casing.

The connection housing enables fluids in a sufficient amount to be circulated outside the wellhead valve.

The tool includes a stinger which is detachably placed in a sealing manner in the connection housing, the stinger communicating with the first pipe bore and with the first perforation.

The stinger thus constitutes a pipe connection between the connection housing and the first perforation.

The stinger includes an expandable packer which is arranged to seal the pipe bore of the inner casing.

At least one of the first pipe bore and the second pipe bore of the connection housing may be in communication with a vessel. Thereby, by means of suitable equipment, fluids for circulation and supply to the well may be pumped down into the well from a vessel.

At least one of the first pipe bore and the second pipe bore may be provided with a valve. Normally, there will be a requirement that a pipe bore in the connection housing be secured with at least one shut-off valve.

The method and the device in accordance with the invention enable a substantial simplification of plugging operations in wells that have the wellhead valve, the BOP, installed, the invention arranging for sufficient circulation capacity through the well. This, in turn, makes it possible for the plugging to be carried out where there are requirements/regulations on the use of a wellhead valve for work in a well. Thus, the plugging may be carried out from a vessel, which means a considerable cost saving compared with using a drilling rig.

In what follows, an example of a preferred method and embodiment is described, which is visualized in the accompanying drawings, in which:

FIG. 1 shows a principle drawing of a subsea well which is provided with a connection housing according to the invention;

FIG. 2 shows the same as FIG. 1, but after a packer has been set and circulation of cleaning fluid may take place; and

FIG. 3 shows the same as FIG. 2, but after a casting substance has been supplied to the well.

In the drawings, the reference numeral 1 indicates a subsea well extending from the seabed 2 and downwards in the ground 4. A typical well 1 includes a conductor pipe 6 with at least one casing 8 and an inner casing 10 which is connected to a production tubing 12 by means of a production packer 14. The areas between a borehole 16 and the different pipes 6, 8, 10 and 12 may be filled, at least partially, with substances such as casting substances and sand. The well 1 is otherwise filled with fluid 20.

A connection housing 22 is arranged between the well 1, that is to say one or more of the casings 6, 8, 10 in the well 1, and a wellhead valve 24. In the figures, the connection housing 22 is shown as one unit but may comprise various components not shown.

A through-going centric bore 26 provides a fluid connection between the wellhead valve 24 and the well 1. A first pipe bore 28 extends from the outside of the connection housing 22 into the bore 26. The first pipe bore 28 communicates with a vessel, not shown, via a first valve 30 and a first pipe connection 32.

A second pipe bore 34 extends from the outside of the connection housing 22 and opens into the well 1. The second pipe bore 34 communicates with the vessel, not shown, via a second valve 36 and a second pipe connection 38.

When the well 1 is to be plugged, a perforation tool 40 is sluiced into the well 1 via the wellhead valve 24 and the connection housing 22. In FIG. 1, the perforation tool 40 is shown in two positions for the perforation of a first perforation 42 and a second perforation 44.

The perforation tool 40 is then pulled out of the well 1 before a stinger 46 is moved into the well 1, see FIG. 2. The stinger 46 is provided with an expanded packer 48 which, after expansion, seals against the inner casing 10. The stinger 46 opens below the packer 48 into the inner casing 10.

Further, the stinger 46 is formed with a boss 50 that fits sealingly in the bore 26 of the connection housing 22. The stinger 46 communicates with the first pipe bore 28.

Circulation fluid for cleaning of an annulus 52 located outside the inner casing 10 may then be circulated via the first pipe connection 32, the first valve 30, the first pipe bore 28, the stinger 46, the pipe bore of the inner casing 10, the first perforation 42, the annulus 52, the second perforation 44, the pipe bore of the inner casing 10, the second pipe bore 34, the second valve 36 and the second pipe connection 38.

After the cleaning operation has been finished, a casting substance 54 is supplied to the well via the flow path mentioned, until a sufficient length of the annulus 52 is filled with the casting substance 54. In the lower portion of the inner casing 10 below the first perforation 42, a packer, not shown, may be arranged to keep the casting substance 54 separated from the fluid 20 in the well 1. Or else, a partition is arranged between the casting substance 54 and the fluid 20 in the well 1 by means of so-called balanced flow.

After the casting substance 54 has set, the well 1 is plugged. The packer 48 may be relieved and pulled out of the well 1 together with the stinger 44. The wellhead valve 24 and the connection housing 22 may then be removed. 

1. A method for plugging a subsea well in which a wellhead valve is arranged at the seabed, the method comprising: moving a perforation tool into the well and perforating a first perforation through the wall of an inner casing; perforating a second perforation in the wall of the inner casing at a considerable distance from the first perforation; filling a casting substance into at least a portion of an annulus in an area between the first perforation and the second perforation; placing a connection housing between the well and the wellhead valve; performing the first perforation in at least the wall of the inner casing at a height level at which the perforation enables access to the wall of a borehole; setting a packer in the inner casing between the two perforations; connecting a stinger to the connection housing, the stinger communicating with the first perforation; circulating fluid through an annulus located outside the inner casing, between the first perforation and the second perforation to clean the annulus; and filling a casting substance into the inner casing below the packer and at least into a portion of the annulus between the first perforation and the second perforation.
 2. A method in accordance with claim 1, further comprising removing the packer and the stinger.
 3. A plugging tool for plugging a subsea well in which a wellhead valve is arranged at the seabed, the tool comprising a connection housing which fits sealingly between the well and the wellhead valve, the connection housing being formed with a through-going axial bore, a first pipe bore extending from the outside of the connection housing into the bore, and a second pipe bore extending from the outside of the connection housing and communicating with the pipe bore of a casing, and a stinger detachably and sealingly positioned in the connection housing, the stinger communicating with the first pipe bore and with the first perforation, and the stinger including an external expandable packer.
 4. The plugging tool in accordance with claim 3, wherein at least one of the first pipe bore and the second pipe bore communicates with a vessel.
 5. The plugging tool in accordance with claim 3, wherein at least one of the first pipe bore and the second pipe bore is provided with a valve. 